Mobile devices, like mobile phones, notebooks, tablet computers, etc. are getting more and more powerful every day. Also, a vast variety of former stationery appliances with high power consumption are getting available in mobile versions. This progress is mainly made possible by the increasing performance of secondary batteries that can now supply the power required to run these devices. Also, the capacity of secondary batteries is now high enough to provide power to these devices for the required amount of time. When the battery charge is depleted, the secondary batteries should be recharged with an appropriate charging device.
Modern secondary batteries, like lithium-ion batteries, which are widely used in small electronic devices, should be charged with special electronic chargers adapted to their charging properties. The charging properties depend on the chemical conversion of the energy inside the secondary battery, where several conditions must be met to ensure the safety of the device, to avoid damage to the battery and to maintain the battery lifetime. Known chargers use direct charging with a limited charging current utilizing an appropriate power supply unit or a power supply independent topology with the constant current/constant voltage method (CC/CV).
With this constant current/constant voltage method, at first batteries are charged with a constant current, avoiding an excessive charging current, especially in the beginning of charging. The constant current phase continues until the end charging voltage is reached. At that time, approximately 70% to 80% of the nominal capacity of the secondary battery is reached. Subsequently, charging commences with constant voltage, thereby ensuring that the end charging voltage is not exceeded. In the constant voltage phase, the charging current decreases gradually with increasing battery charge.
To implement such a method into a charging device, special charging ICs are necessary. For small power requirements, it is common to use integrated charging ICs. The drawback of such charging ICs is a high complexity to provide for the required functionality. This leads to a large chip area, a high number of pins, long time requirements for testing and especially high cost of the device. Another important drawback is a lack of transparency for the developers due to the “black-box” behavior of the charging ICs. This makes it difficult to combine the charging IC with own microcontrollers and/or software because the developer does not have access to the working details of the charging IC.
For low-cost devices, simple dc-converters are used with a shunt resistor for charging a secondary battery. With such a device, for low battery voltage, the dc-converter always delivers its maximum output current to the secondary battery. The charging current is therefore only reducing, when the battery voltage is coming closer to the end voltage. This leads to an excessive heating up of the dc-converter in the beginning of charging and to very high charging currents leading to a severe reduction of battery lifetime.
It is therefore an object of the present invention to provide a simple and low-cost electrical circuit for charging a secondary battery that ensures safety, avoids damage to the secondary battery and maintains the battery lifetime, in particular for household appliances such as hair cutting or removal appliances (shaver, epilator), toothbrushes or skin treatment devices (massage). It is a further object of the present invention to provide a secondary battery over current protection.